In the state of the art, it is known to install shielded cables in buildings for the purpose of constituting a computer network for transmitting data or information. The transmission cables are generally mounted inducting and they are fitted at various points with shielded electrical connectors for connection to computer equipment.
It can sometimes happen that the shielding of such circuits is defective. Such a defect may be due to an "injury" suffered by the shielding, or to a poor electrical connection between the shielding of the electrical connector and the shielding of the cable. A shielding defect is liable to give rise to misfunction of the computer installation by making it more sensitive to interference. The computer installation can thus be disturbed by interference which may be constituted, for example, by fast transients in the electrical power supply, by electrostatic discharges, or by a radiated field or a magnetic field from lightning.
It thus appears to be relatively important to be able to test the transfer impedance or attenuation factor of a shielded cable installed in a building in order to ascertain the ability of the shielding to prevent the magnetic fields and electric fields having any effect. In practice, it is impossible to gain access to the shielding of the cable insofar as the cable is generally contained within ducting.
Document EP-A-0 454 578 which describes a method and apparatus for verifying the effectiveness of the electromagnetic shielding of a conductive cord, does not enable such an object to be achieved. The technique described recommends connecting one end of a conductor of the cord to the shielding of the associated connector via a resistor, while the other end of the conductor is connected to the shielding of the corresponding connector via another resistor. That technique therefore suffers from the drawback of requiring access to both ends of the conductor cord that is to be inspected.
Also known from French patent application FR 2 170 830, are a method and apparatus for inspecting the shielding of a shielded conductor forming a part of cabling constituted by a plurality of conductors. That document teaches isolating the shielding that is to be inspected from ground, and applying an alternating voltage thereto. All of the conductors and shielding of the cabling, other than the shielding under inspection, are grounded. A voltage is picked up from the conductor protected by the shielding, so as to assess the state of the shielding. That technique requires the use of cabling that is of a special type and it also makes it necessary to have access to the various conductors of the cabling.